1. Field of the Invention
The present invention relates to constant-voltage circuits.
2. Description of the Related Art
A constant-voltage circuit configured to supply a constant voltage to a load includes an overcurrent protection circuit, which limits a load current value when the load current has exceeded a rated current in order to protect the inside of the circuit and the load. FIG. 7 is a circuit diagram of a conventional constant-voltage circuit including an overcurrent protection circuit disclosed in Japanese Patent No. 4574902.
First, a constant-voltage circuit 100 shown in FIG. 7 is described. The constant-voltage circuit 100 is configured to generate a constant output voltage VOUT based on an input voltage VDD (power supply voltage) applied to an input terminal IN, and output the output voltage VOUT from an output terminal OUT. Specifically, the output voltage VOUT is divided by a voltage divider circuit 150 including resistors R151 and R152. Then, an error amplifier 130 compares a voltage obtained by the voltage division by the voltage divider circuit 150 (the obtained voltage is hereinafter referred to as a “divided voltage”) with a reference voltage from a reference voltage source 120. Based on a result of the comparison, a gate terminal of an output transistor M110 is controlled.
To be more specific, the output transistor M110 shown in FIG. 7 is configured as a PMOS transistor, and its drain terminal is connected to the output terminal OUT and the voltage divider circuit 150. The divided voltage from the voltage divider circuit 150 is applied to a non-inverting input terminal of the error amplifier 130, and the reference voltage from the reference voltage source 120 is applied to an inverting input terminal of the error amplifier 130. If the divided voltage from the voltage divider circuit 150 is lower than the reference voltage from the reference voltage source 120, a gate voltage VG (M110) of the output transistor M110 decreases in accordance with an output signal from the error amplifier 130. As a result, the output voltage VOUT increases. On the other hand, if the divided voltage from the voltage divider circuit 150 is higher than the reference voltage from the reference voltage source 120, the gate voltage VG (M110) of the output transistor M110 increases in accordance with an output signal from the error amplifier 130. As a result, the output voltage VOUT decreases. As described above, the constant-voltage circuit 100 operates in such a manner as to cause the output voltage VOUT outputted from the output terminal OUT to be constant.
Next, an overcurrent protection circuit 40 shown in FIG. 7 is described. The overcurrent protection circuit 40 includes a first sense transistor M130, a second sense transistor M170, a current detection circuit 70, and a protection circuit 80. It should be noted that the first sense transistor M130 and the second sense transistor M170 shown in FIG. 7 are configured as PMOS transistors. A gate terminal of the first sense transistor M130 and a gate terminal of the second sense transistor M170 are connected to the gate terminal of the output transistor M110. A source terminal of the first sense transistor M130 and a source terminal of the second sense transistor M170 are connected to a source terminal of the output transistor M110.
A drain terminal of the first sense transistor M130 and a drain terminal of the second sense transistor M170 are connected to the current detection circuit 70. Owing to an operation of the current detection circuit 70, which will be described below, a drain voltage VD (M130) of the first sense transistor M130 is controlled to be equal to a drain voltage VD (M110) of the output transistor M110. As a result, a drain current corresponding to the ratio between the gate size of the output transistor M110 and the gate size of the first sense transistor M130 flows through the drain terminal of the first sense transistor M130.
The drain current of the first sense transistor M130 is inputted to the protection circuit 80 via the current detection circuit 70. The protection circuit 80 includes transistors M100 and M200 and resistors R100 and R200, and is configured to control a gate-source voltage VGS (M110) of the output transistor M110 in accordance with the value of the drain current of the first sense transistor M130. It should be noted that the transistor M100 and the transistor M200 included in the protection circuit 80 are configured as a PMOS transistor and an NMOS transistor, respectively. The drain current of the first sense transistor M130 is converted into a voltage by flowing through the resistor R200. The converted voltage is applied to a gate terminal of the transistor M200. If a gate-source voltage VGS (M200) of the transistor M200 exceeds a threshold voltage VTH200 of the transistor M200, then the transistor M200 becomes a conductive state and a current flows through the resistor R100, so that a voltage drop at the resistor R100 increases. As a result, the transistor M100 whose gate terminal is connected to one end of the resistor R100 becomes a conductive state, and the gate voltage VG (M110) of the output transistor M110 becomes equal to a source voltage VS (M110). At the time, the gate-source voltage VGS (M110) of the output transistor M110 becomes zero, and the output transistor M110 becomes a non-conductive state. Consequently, the supply of a current to a load (not shown) connected to the output terminal OUT is stopped. Thus, overcurrent protection by the overcurrent protection circuit 40 is performed in the above-described manner.
Next, the current detection circuit 70 shown in FIG. 7 is described. It should be noted that transistors M704, M706, M708, M709, and M710 included in the current detection circuit 70 are configured as a PMOS transistor, an NMOS transistor, a PMOS transistor, an NMOS transistor, and a PMOS transistor, respectively.
First, assume that the gate size of the first sense transistor M130 and the gate size of the second sense transistor M170 are equal to each other. Since the first sense transistor M130 and the second sense transistor M170 are connected to each other at their source terminals and gate terminals, gate-source voltages VGS (M130) and VGS (M170) of the respective first and second sense transistors M130 and M170 are equal to each other. Accordingly, if drain voltages VD (M130) and VD (M170) of the respective first and second sense transistors M130 and M170 are adjusted to be equal to each other, then drain-source voltages VDS (M130) and VDS (M170) of the respective first and second sense transistors M130 and M170 become equal to each other. At the time, a current flowing through the first sense transistor M130 and a current flowing through the second sense transistor M170 have the same current value.
A source terminal of the transistor M708 is connected to the drain terminal of the second sense transistor M170. A drain terminal of the transistor M706 disposed at the input side of a current mirror circuit is connected to a drain terminal of the transistor M708. A drain terminal of the transistor M710 is connected to a drain terminal of the transistor M709 disposed at the output side of the current mirror circuit. Accordingly, a current flowing into the source terminal of the transistor M708 and a current flowing into a source terminal of the transistor M710 have the same current value.
A source terminal of the transistor M704 is connected to the drain terminal of the first sense transistor M130. A gate terminal of the transistor M704 is connected to gate terminals of the respective transistors M710 and M708. Accordingly, a current flowing into the source terminal of the transistor M704 and the current flowing into the source terminal of the transistor M710 have the same current value.
It is understood from the above description that the currents flowing through the respective transistors 704, 708, and 710 are equal to each other. Also, gate-source voltages VGS (M704) and VGS (M710) of the respective transistors M704 and M710 are equal to each other. Here, the source terminal of the transistor M710 is connected to the output terminal OUT, and a source voltage VS (M704) of the transistor M704 is equal to the output voltage VOUT. Accordingly, it is understood that the ratio between the value of a current flowing through the drain terminal of the first sense transistor M130 and the value of a current flowing through the drain terminal of the output transistor M110 is equal to the ratio between the gate size of the first sense transistor M130 and the gate size of the output transistor M110.